avajs/ava · error · TypeError

The extensions option must be an array

Error message

The extensions option must be an array

What it means

The `extensions` option tells AVA which file extensions to treat as test files. It must be an array of strings; if `resolveExtensions` receives a configured value that is not an array, it throws this TypeError immediately.

Source

Thrown at lib/extensions.js:18

export default function resolveExtensions(configuredExtensions, providers = []) {
	// Combine all extensions possible for testing. Remove duplicate extensions.
	const duplicates = new Set();
	const seen = new Set();

	const combine = extensions => {
		for (const ext of extensions) {
			if (seen.has(ext)) {
				duplicates.add(ext);
			} else {
				seen.add(ext);
			}
		}
	};

	if (configuredExtensions !== undefined) {
		if (!Array.isArray(configuredExtensions)) {
			throw new TypeError('The extensions option must be an array');
		}

		combine(configuredExtensions);
	}

	for (const {main} of providers) {
		combine(main.extensions);
	}

	if (duplicates.size > 0) {
		throw new Error(`Unexpected duplicate extensions in options: ’${[...duplicates].join('’, ’')}’.`);
	}

	// Unless the default was used by providers, as long as the extensions aren't explicitly set, set the default.
	if (configuredExtensions === undefined) {
		if (!seen.has('mjs')) {
			seen.add('mjs');
		}

View on GitHub (pinned to bbfd946322)

Solutions

1. Change the value to an array: `extensions: ['js', 'mjs']`.
2. If a single extension, still wrap it: `extensions: ['ts']`.
3. Split comma-separated strings: `'js,mjs'.split(',')`.
4. Add Array.isArray validation in a config factory before returning.

Example fix

// before
export default {extensions: 'js'};
// after
export default {extensions: ['js']};

if (config.extensions !== undefined && !Array.isArray(config.extensions)) {
  throw new TypeError('extensions must be an array');
}

const isValidExtensions = v => v === undefined || (Array.isArray(v) && v.every(e => typeof e === 'string'));

try {
  await run();
} catch (err) {
  if (err instanceof TypeError && err.message === 'The extensions option must be an array') {
    console.error('Wrap the extensions value in an array in your AVA config');
    process.exitCode = 1;
  } else throw err;
}

When it happens

Trigger: Passing `extensions: 'js'` (a string), `extensions: {main: ['js']}`, or any non-array value in ava.config.js/package.json or to the programmatic API, instead of an array like `extensions: ['js', 'mjs']`.

Common situations: Config typo where quotes wrap a comma list ('js, mjs'); object-shaped config copied from another tool; a config factory returning a wrong type after refactoring.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

• The ’environmentVariables’ configuration must be an object c
• ${fileForErrorMessage} must export a plain object or factory
• Unexpected duplicate extensions in options: ’${[...duplicate
• The ’files’ configuration must be an array containing glob p
• The ’watchMode.ignoreChanges’ configuration must be an array

AI-assisted analysis of avajs/ava@bbfd946322 (2026-09-02). Data as JSON: /api/errors/efadb0a438a3fe0c. Report an issue: GitHub.